Series and parallel resistor combinations in real designs
Combining resistors is one of the simplest and most common tasks in circuit design. You may need a non-standard value, want to spread power across multiple parts, or adjust an existing design without changing the PCB.
This calculator helps you quickly find the equivalent resistance for:
- Two or more resistors in series
- Two or more resistors in parallel
- Mixed combinations (series of parallels, parallel of series groups)
Use it when tuning analog circuits, building load banks, or checking how modifications will affect current and voltage distribution.
Series resistor rule
When resistors are connected end-to-end in series, the same current flows through each one and the total resistance is the sum:
Rtotal = R1 + R2 + … + Rn
Example:
- R1 = 1 kΩ
- R2 = 2.2 kΩ
- R3 = 3.3 kΩ
Rtotal = 1 kΩ + 2.2 kΩ + 3.3 kΩ = 6.5 kΩ
Series combinations are typically used to:
- Obtain a higher resistance value
- Spread high voltage across multiple parts
- Fine-tune values using small trim resistors
Parallel resistor rule
For parallel resistors, the voltage across each branch is the same, and conductances add. The total resistance is given by:
1 / Rtotal = 1 / R1 + 1 / R2 + … + 1 / Rn
For two resistors, a useful shortcut is:
Rtotal = (R1 × R2) / (R1 + R2)
Example (two in parallel):
- R1 = 100 Ω
- R2 = 200 Ω
Rtotal = (100 × 200) / (100 + 200) = 20,000 / 300 ≈ 66.7 Ω
Parallel combinations are commonly used to:
- Obtain an effective resistance lower than any individual resistor
- Share power dissipation across multiple parts
- Use standard values to approximate a non-standard low resistance
Quick reference table
The table below shows some common two-resistor parallel combinations:
| R1 | R2 | Rtotal (parallel) | Notes |
|---|---|---|---|
| 100 Ω | 100 Ω | 50 Ω | Equal values → half the resistance |
| 220 Ω | 220 Ω | 110 Ω | Useful to double power rating |
| 1 kΩ | 2 kΩ | ≈ 666 Ω | Non-integer combination for fine tuning |
| 10 kΩ | 10 kΩ | 5 kΩ | Common for analog input scaling |
Using the Parallel and Series Resistor Calculator
- Choose configuration: Select whether you are combining resistors in series, in parallel, or building a small mixed network (for example, two parallel branches in series).
- Enter resistor values: Type the resistance of each resistor. The calculator supports ohms, kilohms and megohms if your UI allows unit selection.
- Review equivalent resistance: The tool displays Rtotal. In some modes, it can also show individual branch currents or power if a supply voltage is specified.
- Apply to your design: Use the result to select practical component combinations, check that ratings are met, or adjust existing circuits without changing the PCB.
Design tips for combining resistors
- Share power carefully in parallel: The lowest-tolerance or lowest-resistance part tends to run hotter and may hog more current. Use equal values and tight tolerances for better sharing.
- Mind voltage ratings in series chains: For high-voltage dividers, ensure each resistor’s maximum working voltage is not exceeded and distribute values to balance stress.
- Use standard series smartly: E12/E24/E96 series values can be combined to approximate almost any target resistance. The calculator shows the exact equivalent so you can judge the error.
- Consider temperature coefficients: In precision networks, mixing different technologies (e.g. metal film + carbon) may introduce drift. Use matched parts where stability matters.
With this calculator, you can move from target resistance to real-world combinations in seconds, making it easier to reuse inventory parts, build load banks, and fine-tune both low-power and industrial circuits.
